Hi,
It's a constant pain with Palo and I can't believe they still don't have a solution to provide automated gateway redundancy but that is the reality (unless you have A/A firewalls which almost no one can do).
In your scenario you only need the IP on the interface on the second firewall if that firewall needs to route over that interface under BAU. If it does not, you can simply have the interface with the real IP on your primary firewall and no IP on the secondary firewall, then create it on the secondary firewall in a failover scenario.
If dealing with multiple interfaces you can script it to create/move the IP a little faster.
Another way I've dealt with this (when using more upstream or downstream routers is not an option) is to create all your required interfaces in a second VRF on the second firewall, all with the real IP address. If you have a failure, just enable routing between the VRFs to "liven up" all the interfaces that are usually offline. This could be static if your subnetting allows for a single route in each direction, or you could use a dynamic protocol and just have it disabled under BAU. This works really well for DR / BCP environments or when dealing with a lot of interfaces that would make manually moving IPs very tedious.
Not saying it's a solution - just another way of working around the lack of functionality...
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